Recent advances in IC design and fabrication make possible the integration of various types of integrated circuits, for example, digital and analog circuits on a same IC chip. The high degree of integration causes high noise coupling between the integrated circuits. Particularly, analog circuits are easily affected by the noise generated in digital circuits. This significantly limits the performance of analog circuits, such as analog to digital converters that are extremely sensitive to noise at the inputs. Besides the noise interference between digital circuits and analog circuits, noise interference also exists between digital circuit components. FIG. 1 illustrates a noise path between a digital circuit region 2 and an analog circuit region 3. Arrows 4, 5 and 6 symbolize one of the noise paths in substrate 1.
A significant portion of the noise coupling occurs in substrates. Therefore, various methods have been developed to break the noise paths in substrates. One commonly used method is to form isolation layers in the substrate. As is shown in FIG. 1, an isolation layer 7 is formed in semiconductor substrate 1, breaking the noise path between circuit regions 2 and 3. Isolation layer 7 is typically formed of dielectric materials. One example of isolation layer 7 is a deep trench isolation. To form a deep trench isolation, trenches with nearly vertical sides are etched between circuits and then filled with dielectric materials. However, even deep trench isolations are not satisfactory when full isolation between the circuits is required. This is particularly true when high-speed analog circuits are involved.
Another known method is the placement of guard rings in the substrates between the circuits to be isolated. As illustrated in FIG. 2, a p+ guard ring 8 is formed in a p− substrate 1. The guard ring 8 is grounded at node 9. Therefore, a low resistivity path for the substrate noise is created. The noise generated by circuits 2 and/or 3 is more likely to take the low resistivity path to guard ring 8 than to another circuit region, which path has a higher resistivity.
Similar to the structure shown in FIG. 2, conductive deep trenches have also been used. These are more effective at isolating substrate noises than guard rings and shallow trenches. However, the effectiveness of deep trenches is related to their depth, thus their effectiveness is limited. New methods are therefore needed.